Phospholipids accelerate factor IX activation by surface bound factor XIa.

Activation of bovine factor IX by surface bound factor XIa which was generated either by activation of human citrated factor IX deficient plasma or a mixture of purified human factors XII, high molecular weight kininogen (HMWK) and XI in glass tubes, is accelerated by cephalin. Human brain cephalin in dilutions ranging from 1:5 to 1:500 was studied for its effect on the activation of factor IX in concentrations of 1.0 u/ml and 16 u/ml. Cephalin dilutions from 1:5 to 1:30 accelerated the activation of the concentrated factor IX sample two- to threefold. Protein cleavage of this factor IX sample in the presence of 1:30 cephalin occurred twice as fast as in the absence of cephalin. Activation of the dilute factor IX sample (1.0 u/ml) was most effectively accelerated by cephalin in dilutions from 1:30 to 1:250. In all experiments the presence of phospholipid led to an increased factor IX cleavage concomitantly with faster generation of factor IXa activity. The results demonstrate that phospholipids actively participate in blood coagulation at an earlier stage than previously described.